Synthesis and optical properties of binuclear gold(I) complexes with bridging phosphine ligands: Luminescence from intraligand and metal-centered excited states

Article abstract of DOI:10.1016/j.ica.2003.09.020

The complexes Au₂Cl₂(P-P) with P-P = biphep (2,2′-bis(diphenylphosphino)-1,1′-biphenyl), binap (2,2′-bis(diphenylphosphino)-1,1′-binaphthyl) and xantphos (9,9′-dimethyl-4,5-bis(diphenyl-phosphino)xanthene), were prepared and characterized by elemental analysis and ESI-MS. The solid compounds show a r.t. phosphorescence. While the binap complex emits from an intraligand (IL) triplet, the luminescence of the biphep complex originates from a metal-centered (MC) triplet which is presumably lowered by gold-gold interaction. The xantphos complex displays a dual phosphorescence. In this case, the emitting triplets are of the IL and MC type.

Full text of DOI:10.1016/j.ica.2003.09.020

Inorganica Chimica Acta 357 (2004) 1309–1312
www.elsevier.com/locate/ica
Note
Synthesis and optical properties of binuclear gold(I) complexes
with bridging phosphine ligands: luminescence from intraligand
and metal-centered excited states
*
Valeri Pawlowski, Horst Kunkely, Arnd Vogler
Institut f€ur Anorganische Chemie, Universit€at Regensburg, Universitaetsstrasse 31, D-93040 Regensburg, Germany
Received 15 August 2003; accepted 3 September 2003
Abstract
The complexes Au2Cl2(P-P) with P-P ¼ biphep (2,20-bis(diphenylphosphino)-1,10-biphenyl), binap (2,20-bis(diphenylphosphino)-
1,10-binaphthyl) and xantphos (9,90-dimethyl-4,5-bis(diphenyl-phosphino)xanthene), were prepared and characterized by elemental
analysis and ESI-MS. The solid compounds show a r.t. phosphorescence. While the binap complex emits from an intraligand (IL)
triplet, the luminescence of the biphep complex originates from a metal-centered (MC) triplet which is presumably lowered by gold–
gold interaction. The xantphos complex displays a dual phosphorescence. In this case, the emitting triplets are of the IL and MC type.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Gold complexes; Phosphine complexes; Luminescence
1. Introduction
matic substituents became available. While they were
designed for applications in catalysis they might also
contribute to gain more insight into the excited state
properties of gold(I) phosphine complexes. We exlored
this possibility and selected the bidentate phosphines
A variety of gold(I) compounds has been shown to be
luminescent under ambient conditions [1–3]. Binuclear
Au(I) complexes which contain bidentate phosphines as
bridging ligands [4] constitute an important subclass of
emissive gold(I) compounds. In this case the lumines-
cence frequently originates from metal-centered (MC)
excited states which are modified by gold–gold interac-
tion. This applies in particular to complexes with alkyl
substituents at phosphorus [5–8]. However, bidentate
phosphines carry often chromophores including aro-
matic groups. Such phosphines have available intrali-
gand (IL) states at relatively low energies [9,10]. The
presence of MC and IL states at comparable energies
complicates the emission behavior. On the other side, in
favorable cases the IL emission is structured and re-
sembles that of the free ligand. The IL emission can then
be easily identified and may be even used as luminescent
marker. Recently, new bidentate phosphines with aro-
2,20-bis(diphenylphosphino)-1,10-biphenyl
2,20-bis(diphenylphosphino)-1,10-bi-naphthyl
(biphep),
(binap)
and 9,90-dimethyl-4,5-bis(diphenylphosphino)xanthene
(xantphos) for the present study.
The complex Au2Cl2(biphep) has been previously
sythesized by a complicated procedure [11]. However, it
was expected that Au(I) complexes of biphep, binap and
xantphos are accessible by simply reacting Au(SMe2)Cl
with the corresponding diphosphine.
* Corresponding author. Tel.: +49-941-9434485; fax: +49-941-
9434488.
E-mail address: arnd.vogler@chemie.uniregensburg.de (A. Vogler).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2003.09.020

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V. Pawlowski et al. / Inorganica Chimica Acta 357 (2004) 1309–1312
2. Experimental
2.1. Materials
sis. Au2Cl2(biphep) has been synthesized previously [11].
Au2Cl2(binap) and Au2Cl2(xantphos) were further
characterized by ESI-MS. The +ESI spectrum of
Au2Cl2(binap) in CH3CN shows peaks at m=z ¼ 1051,
1052, 1053, 1054 and 1055 which correspond to the loss
of one chloride ligand. Additional peaks at m=z ¼ 819,
820, 821 and 822 are caused by [Au(binap)]þ which was
generated as a further fragmentation product. Analo-
gous results were obtained for Au2Cl2(xantphos) in
CH3CN. The +ESI-MS of [Au2Cl2(xantphos)] shows
peaks at m=z ¼ 1007, 1008, 1009, 1010, 1011 for
[Au2Cl2(xantphos)]þ and at 775, 776, 777 and 778 for
[Au(xanthphos)]þ. In all cases the isotope distribution
matches the calculated data.
The free biphep ligand in CH3CN or ethanol exhibits
a broad absorption at kmax ¼ 270 nm, a r.t. fluorescence
at kmax ¼ 384 nm and a structureless phosphorescence at
kmax ¼ 490 nm which appears only at low temperatures
(ethanol, 77 K). The spectral data of binap have been
previously reported [12]. The free xantphos ligand in
ethanol displays an absorption at kmax ¼ 259 nm, and a
lowtemperature phosphorescence at kmax ¼ 475 nm
(ethanol, 77 K).
The electronic spectrum of Au2Cl2(biphep) in
CH3CN (Fig. 1) shows an absorption which increases
towards shorter wavelength. Shoulders appear at
k ¼ 275 nm (e ¼ 10950 Mꢀ1 cmꢀ1), 268 (14 300) and 262
(16 700). In solution the complex is not luminescent, but
in the solid state at r.t. a greenyellow emission occurs
(Fig. 1) at kmax ¼ 580 nm (kexc ¼ 275 nm). The complex
Au2Cl2(binap) exhibits absorptions (Fig. 2) at
kmax ¼ 337 nm (3300) and 230 nm (85 300) and shoul-
ders at 322 and 292 nm. Again, the complex is not lu-
minescent in solution, but the solid compound shows a
green r.t. emission (Fig. 2) with structural features at
k ¼ 502, 535 and 570 nm (kexc ¼ 335 nm). In the ab-
sorption spectrum of Au2Cl2(xantphos) in CH3CN (Fig.
3) two shoulders at 276 nm (20 430) and 270 nm
(21 300), and a maximum at 256 nm (24 040) are ob-
Solvents used for spectroscopic measurements were
of spectrograde quality. The compounds Au(SMe2)Cl,
biphep, binap and xantphos were commercially avail-
able (Aldrich, Strem) and used without further purifi-
cation.
The complexes Au2Cl2(P-P) were obtained by the
following procedures.
2.1.1. Au2Cl2(biphep)
A mixture of Au(SMe2)Cl (430 mg, 1.46 mmol) and
biphep (390 mg, 0.73 mmol) in 40 ml CH3CN/CH2Cl2
(1:1) was stirred under argon at r.t. for 1.5 h. A white
material precipitated. It was collected by filtration, wa-
shed with ether and dried over silical gel, yield: 670 mg
(68%). Anal. Calc. (Found): C, 43.79 (43.89); H, 2.86
(3.17); Cl, 7.18 (7.13%).
2.1.2. Au2Cl2(binap)
To a suspension of binap (160 mg, 0.25 mmol) in 20
ml CH3CN/CH2Cl2 (1:1) under argon was added
Au(SMe2)Cl (150 mg, 0.5 mmol). A clear solution was
formed under strirring. After 30 min a white material
started to precipitate. Strirring at r.t. was continued for
2 h. The precipitate was collected by filtration, washed
with ether and dried over silica gel, yield: 175 mg (64%).
Anal. Calc. (Found): C, 48.59 (47.99); H, 2.97 (3.16); Cl
6.52 (6.30%).
2.1.3. Au2Cl2(xantphos)
A mixture of Au(SMe2)Cl (300 mg, 1 mmol) and
xantphos (290 mg, 0.5 mmol) in 30 ml CH3CN/CH2Cl2
(1:1) was stirred under argon at r.t. for 1 h. A white
precipitate was formed, collected by filtration, washed
with ether and dried over silica gel, yield: 475 mg (91%).
Anal. Calc. (Found): C, 44.89 (43.40); H, 3.09 (3.08%).
2.2. Instrumentation
Absorption spectra were measured with a Shimadzu
2100 spectrophotometer. Emission spectra were recorded
on a Hitachi 850 spectrofluorometer equipped with a
Hamamatsu 928 photomultiplier for measurements up to
900 nm. The mass spectra were obtained using a TSQ
7000 Finnigan Thermoquest mass spectrometer.
3. Results
The complexes Au2Cl2(P-P) with P-P ¼ biphep, binap
and xantphos were prepared by the reaction of Au(S-
Me2)Cl with P-P. They were obtained as white solids.
Their composition were confirmed by elemental analy-
Fig. 1. Electronic absorption (a) and emission (e) spectrum of
Au2Cl2(biphep) at room temperature. Absorption: 3.11 ꢁ 10ꢀ5 M in
CH3CN, 1-cm cell. Emission: solid, kexc ¼ 275 nm, intensity in arbi-
trary units.

V. Pawlowski et al. / Inorganica Chimica Acta 357 (2004) 1309–1312
1311
It is assumed that Au2Cl2(binap) and Au2Cl2(xantphos)
have analogous structures. All three complexes are lu-
minescent at r.t., but only in the solid state. These
emissions certainly originate from triplet states owing to
the heavy atom effect of gold [1–3]. In the case of
Au2Cl2(binap) it is quite clear that the emitting state is
of the IL (binap) type because the emission (Fig. 2)
closely resembles the phosphorescence of the free binap
ligand [12]. The IL assignment is facilitated by the ap-
pearance of a structured emission for the complex as
well as for the free ligand.
Unfortunately, the phosphorescence of Au2Cl2(bip-
hep) (Fig. 1) and of the free biphep ligand is not struc-
tured. However, the complex emits at distinctly longer
wavelength (kmax ¼ 580 nm) than the ligand (490 nm).
Accordingly, it is suggested that the phosphorescence of
the complex does not come from an IL (biphep) state. It
is assumed that the emission originates from a MC (ds,
dp) excited state which is lowered in energy by gold–
gold interaction [5–8]. Indeed it has been shown that the
gold–gold distance of the closely related complex
Au2I2(bis-2,20-diethylphosphino-1,10-biphenyl) amounts
Fig. 2. Electronic absorption (a) and emission (e) spectrum of
Au2Cl2(binap) at room temperature. Absorption: 8.97 ꢁ 10ꢀ6 M in
CH3CN, 1-cm cell. Emission: solid, kexc ¼ 335 nm, intensity in arbi-
trary units.
ꢀ
to 3.167 A and is thus indicative of a weak metal–metal
interaction [11]. In the case of Au2Cl2(binap) the steric
requirements of the binap ligand could prevent such a
close approach of both gold atoms. Accordingly, the
MC state occurs at higher energies well above the
emitting IL state.
The complex Au2Cl2(xantphos) shows two emissions
(Fig. 3). The phosphorescence at 480 nm is assumed to
originate from the xantphos IL state since the lumines-
cence appears at an energy very close to that of the
phosphorescence of the free xantphos ligand. The longer
wavelength emission of the complex at kmax ¼ 613 nm is
assumed to come from a MC triplet modified by gold–
gold interaction.
In conclusion, solid complexes of the type Au2Cl2(P-
P) which contain the bridging phosphines biphep, binap
and xantphos show a room temperature phosphores-
cence which originates from MC and/or IL (P-P)
triplets.
Fig. 3. Electronic absorption (a) and emission (e) spectrum of
Au2Cl2(xantphos) at room temperature. Absorption: 1.51 ꢁ 10ꢀ5 M in
CH3CN, 1-cm cell. Emission: solid, kexc ¼ 300 nm, intensity in arbi-
trary units.
served. While in solution the complex does not emit,
solid Au2Cl2(xantphos) displays a r.t. luminescence (Fig.
3) which consists of two distinct bands. A maximum
appears at kmax ¼ 613 nm and a shoulder at 480 nm
(kexc ¼ 300 nm). In all cases the excitation spectra match
roughly the absorption spectra.
4. Discussion
Acknowledgements
Various complexes of the type Au2Cl2(2,20-diph-
osphino-1,10-biphenyl) including Au2Cl2(biphep) have
been prepared and structurally characterized [11].
Financial support by BASF is gratefully acknowl-
edged.
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